The present invention relates generally to differential gear mechanisms, and more particularly, to such mechanisms of the type commonly referred to as "locking differentials".
Differential gear mechanisms of the type to which the present invention applies are broadly referred to as "limited slip differentials" and typically include a clutch pack which is operable to limit or retard differentiating action between the output gears. More specifically, however, the present invention is intended for use on limited slip differentials of the type referred to as "locking differentials", and will be described in connection therewith. In a locking differential, means are provided for engaging or locking the clutch set, rather than permitting it to slip, to substantially reduce the amount of differentiating action permitted between the output gears.
As one result of the general trend toward downsizing of automobiles, it has been necessary to similarly downsize many of the vehicle components, including locking differentials. The development and testing of smaller locking differentials has shown the necessity for closer tolerances in order to achieve the same levels of performance as were previously achieved on the larger units. With respect to the present invention, the term "level of performance" refers to the ability of a locking differential unit to engage the clutch pack (lock-up) within a specified time range. For example, in one commercial embodiment of a locking differential, once the "low traction" wheel begins to spin out until, and locking action is initiated, until that wheel stops spinning (i.e., the time required for the differential to lock) should be not less than 0.1 seconds, and not more than 0.5 seconds.
If the lock-up occurs in less than the specified minimum time (e.g., 0.1 seconds), there are two possible, undesirable conditions which can occur. First, if there is too rapid a build-up of torque in the clutch, impact-type forces are experienced, causing excessive stress and objectionable noise. A second possibility is that an engagement which occurs too abruptly will result in "cycling", a condition in which, due to the mass-spring rate of the drivetrain and tires, the unit alternately and rapidly engages and disengages. This condition is quite objectionable to the driver and can result in much more rapid fatigue of various system components.
On the other hand, if the engagement requires longer than the specified time (e.g., 0.5 seconds), a substantial amount of torque will continue to be transmitted to the clutch pack in the form of heat. At best, the result will be an undesirable overheating of the clutch pack, which in turn would make successive engagements even slower thus causing more heat. At worst, the excessive heat will damage frictional engaging surfaces (which could even cause clutch plates to "weld" together and then be torn apart) and start to destroy the oil within the system. This would severely limit the capacity of future engagements.